How to limit speed in Nginx depending on server channel load?

Question

Nginx

I use the limit_rate directive. I would like to improve a little and get a regular expression like this:
if ($bandthwich_load) {
set $limit_rate 128k;
}
Tell me how to bind a condition to a high load on the channel (excluding simultaneous peaks)? Suppose, when loading a channel at 70Mbit / s, do we limit it to 256k? at 90Mbit/s - 128k...